#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(x,y,z) for(int x=(y);x<=(z);x++)
#define per(x,y,z) for(int x=(y);x>=(z);x--)
#define mid (l+r>>1LL)
#define repn(x) rep(x,1,n)
#define repm(x) rep(x,1,m)
#define pb push_back
#define Pi pair<int,int>
#define e(x) for(int i=h[x],y=to[i];i;i=nxt[i],y=to[i])
#define E(x) for(auto y:p[x])
inline int read(){int s=0,w=1;char c=getchar();while(c<48||c>57){if(c=='-')w=-1;c=getchar();}while(c>=48&&c<=57)s=(s<<1)+(s<<3)+c-48,c=getchar();return s*w;}
const int N=1e6+5,M=1e6+5,inf=(1LL<<31)-1,mod=1e9+7;
const ll llf=1e18;
inline void add(int &a,int b){((a+=b)>=mod) and (a-=mod);}
inline int Add(int a,int b){return add(a,b),a;}
inline int mul(int a,int b){return 1LL*a*b%mod;}
inline void Mul(int &a,int b){a=mul(a,b);}
inline void red(int &a,int b){add(a,mod-b);}
inline int Red(int a,int b){return red(a,b),a;}
inline int qp(int a,int b){if(!b)return 1;int c=qp(a,b>>1);Mul(c,c);if(b&1)Mul(c,a);return c;}
inline int INV(int x){return qp(x,mod-2);}
int n,k,a[N],m;
inline void Main(){
    n=read(),k=read(),m=n-k;
    repn(i)a[i]=read();
    int ans=a[1];
    if(n==1)return cout <<ans,void();
    int W=mul(m-1,INV(n-1));// W 的概率被选为关键点
    add(ans,mul(a[2],W));
    red(ans,mul(a[2],Red(1,W)));
    if(n==2)return cout <<ans,void();
    int W2=mul(m-1,INV(n-2));
    if(m-1>n-2)W2=0;
    rep(i,3,n){
        add(ans,mul(a[i],W));
        red(ans,mul(a[i],mul(Red(1,W),W2)));
    }
    cout <<ans;
}
signed main(){
    freopen("game.in","r",stdin);
    freopen("game.out","w",stdout);
    int T=1;
    while(T--)Main();
    return 0;
}